China has installed a silent alarm system inside every house in a border town as part of its strengthened crackdown on fugitives from North Korea, a report said Friday.

The system is designed to let residents secretly send a signal to police if North Korean escapees come to their houses and ask for help, South Korea's Yonhap news agency said.

It can transmit dialogue between the owner of a house and visitors, and Chinese authorities plan to expand it into other areas bordering the North, the agency said.

"If you push the red button on the wall, a signal goes directly to a police station," Yonhap quoted one man as saying.

The man said he saw the device during a recent trip to his relative in the Yanbian border area in northeastern Jilin province.

Yonhap said China had stepped up a crackdown in border areas since South Korea criticised its repatriation of dozens of North Korean refugees in February and this month.

Almost all those fleeing the North cross first to China, where they face repatriation if caught. Many hide out and then travel on to Southeast Asian nations before flying to the South for resettlement.

Seoul has repeatedly urged Beijing to treat fugitives from the North as refugees and not to send them back, saying they face harsh punishment. China says they are economic migrants and not refugees deserving protection.

The UN refugee agency and rights watchdog Amnesty International have also urged Beijing not to send refugees back. Amnesty says returnees are sent to labour camps where they are subject to torture.